Article written by Charles Torok, Chemist, Analytical Products Group, Inc.
Complete article from Edition 32 APG eNewsletter
ICP-OES is a powerful and efficient tool to determine the concentration of metals in a solution. Under ideal conditions, each line of an element will have a typical Gaussian curve as seen in Figure 1. The baseline can easily be fixed at the two tails of the curve, and a method can be used repeatedly with minimal fine-tuning.
Multi-element analysis introduces the complication of inter-elemental interferences. For most elements, at least one line can be found that is not greatly affected by any other elements, and analysis can be easily carried out. On the other hand, some elements do not have an easily isolated line that can be used for analysis. An example of this is Thallium, in which the USEPA Method 6010B recommended line at 190.864 has strong interferences from Vanadium to the right of the central peak as in Figure 2.
This creates a situation in which the baseline of the line must be carefully defined and should be checked each time Thallium is being tested for. The choice for placing the baseline on the right is either at positions C or D in Figure 2. At position D, the far end of the tail allows for a zero, but in a case of very high Vanadium concentration may be unusable. As seen in Figure 3, the D position is highly inflated because of the relatively high concentration of Vanadium. At position C, the pixels of the instrument may not allow for the placing of the baseline at the bottom of the trough. This may give a higher baseline than desirable because of the averaging of the value at that pixel.
A similar situation occurs for the left side of the central Thallium peak with position B being in the trough between the central Thallium signal and secondary Thallium signal and position A being farther away from the signal.
Choosing the correct combination of left and right placement of the baseline is an exercise to be done each time the element Thallium is tested. This requires the use of QC's with the Calibration curve to find the most optimal placement for the testing as it is performed. USEPA Method 6010B offers an in-depth discussion of inter-element corrections and how they are to be used. Inter-element corrections will vary between instruments due to configurations and the manufacturer. Inter-element interferences should be evaluated on a routine basis following recommendations from the reference method and manufacturer's instructions.
Note: All figures in this article were acquired using a Thermo Jarrell Ash IRIS Advantage using TEVA version 1.4.0.
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